memory leak in getDeviceProperty

[paulz]

we are getting a lot of strings leaking from running monitor just for a few seconds:

leaks Report Version: 4.0
Process 57082: 153864 nodes malloced for 26207 KB
Process 57082: 1036 leaks for 52400 total leaked bytes.
999 (49.9K) ROOT LEAK: CFString

memory graph helped to find the location:

USBDeviceSwift/Sources/SerialDeviceMonitor.swift

Line 31 in 4f9ce75

return propValue?.takeUnretainedValue()

to fix we used:
return propValue?.takeRetainedValue()